swarming: Add more documentation to task messages
swarming: Add more documentation to task messages

In particular note right away that NewTaskRequest.dimensions is an AND: all
values specified must match.

R=vadimsh@chromium.org
BUG=728124

Review-Url: https://codereview.chromium.org/2931063002

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py